Potassium (K) is being gradually removed from Australia’s cropping soils. Since 2002, average crop yields have increased but K fertiliser use has remained static at around 180 kt K/yr. Soil K reserves declined an average of 4 kg K/ha/yr over the last 20 years. Most cropping paddocks have received no K fertiliser over that period.

In the higher rainfall Southern grain growing region, An IPNI survey found that:

less than 10% of fields received K

where applied, the average K application rate was 75 kg K/ha

over all paddocks K removal rates ranged up to 250 kg K/ha

only 6 of the 60 paddocks that applied K had a K surplus

Does potassium rundown matter?

Sandy soils are more likely to be potassium deficient. In heavier cropping soils, K reserves are likely to be sufficient to supply crops for many more seasons at current removal rates. Eventually though, K rundown will reduce paddock fertility.

Using NPIs to track potassium

Nutrient Performance Indicators (NPIs) track trends in nutrient management. They can help understand if soil K reserves are building up or running down, and in understanding how effective K applications have been.

Partial Nutrient Balance-K

PNB-K = K removed (kg/ha) / K applied (kg/ha)

PNB-K indicates whether a crop left the paddock with higher, lower or stable K reserves. Where soil test values are near critical, the goal is for average PNB-K to approach 0.9–1, to maintain soil fertility. Values above 1 indicate run down of soil K.

It is important to view the PNB-K balance in relation to K soil test levels. Many soils have high K reserves, but on low K soils depletion can cause deficiencies over time. Interpret PNB-K as a trend over years to see if soil reserves are increasing or declining.

In Southern Australian grain farms the median PNB-K was 3.0, most of which did not apply any K. On farms where K was applied (mainly in south-west Victoria) – most still had PNB-K >1. Soil K was still being depleted even where K fertiliser was being applied.

Partial Factor Productivity-K

PFP-K = grain produced (kg) / K applied (kg)

PFP-K describes the grain yield benefit from K fertiliser. Higher PFP values mean the crop yield responded strongly to the fertiliser. The normal range for PFP-K is 76–200. PFP-K can only be calculated where K fertiliser is applied.

Regional use of fertiliser K targets paddocks with clear deficiencies, where crop response to fertiliser K can be strong. PFP-K could be calculated for only a small number of farms, since K was applied to only 10% of the survey paddocks. On these farms the average PFP-K exceeded the reference range at 350 kg grain/kg K.

The high PFP-K recorded on the few farms that used K fertiliser does not mean applying K is likely to be profitable. K is not a limiting nutrient in many locations.